How to handle transient dependencies

[m-kuhn]

At the moment PIP will install all dependencies of packages that it is installing. This is of course a useful feature, but leads to duplication if some of the dependencies are already installed (e.g. a package relying on numpy, matplotlib etc. that are probably already installed in the environment). How about only installing the specific dependencies specified in the plugin requirements.txt, without sub-dependencies (not a perfect solution....).

Originally posted by @angusmcb in #14 (comment)